1. Field of the Invention
The invention generally relates to Packet Data Protocol (PDP) context handling, and more particularly, to PDP context handling for an emergency attached Mobile Station (MS).
2. Description of the Related Art
In current wireless communication technology, an MS may be attached for emergency bearer services to make emergency Packet-Switched calls such as IP Multimedia Subsystem (IMS) calls. An MS is attached for emergency bearer services if it has successfully completed an attachment for emergency bearer services or if it has only a Public Data Network (PDN) connection for emergency bearer services established. When the MS is in a limited service state, it may be only allowed to make emergency calls (e.g. an emergency IMS call). The limited service state is entered when the MS is unable to find a suitable cell to camp on, a Subscriber Identity Module (U)SIM card is not inserted therein, or if the MS receives certain responses to a Location Registration request (e.g., “illegal MS”). In the above cases, the MS attempts to camp on a cell irrespective of the Public land mobile network (PLMN) identity.
According to 3GPP TS 23.216 V9.2.0, a PDP context for an IMS call for an emergency bearer service uses traffic class=conversational and SSD=“speech”. When the Radio Access Bearer (RAB) of a PDP context for a call with the traffic class=conversational is released by the Radio Access Network (RAN), the maximum uplink bit rate and the maximum downlink bit rate are downgraded to 0 kbit/sec in both the MS and the network. To re-establish an RAB for the active PDP context that is not associated with an RAB, the MS may initiate a re-establishment of RABs by using a Service Request (Service Type=Data) message. For the PDP context with the maximum uplink bit rate and the maximum downlink bit rate of 0 kbit/sec, the network (or the Serving GPRS Support Node, SGSN) shall not establish an RAB. Instead, the MS shall perform a MS-initiated PDP Context Modification to modify the associated bearer resources (i.e. the maximum uplink bit rate and the maximum downlink bit rate) or a Deactivation procedure to deactivate the PDP context.
However, according to 3GPP TS 23.060 V9.5.0, when there is a PDN connection (an active PDP context) for emergency bearer services established, the MS shall not request a modification of the bearer resources (PDP context modification procedure) for the PDN connection. In this case, when the associated RAB of a PDP context for emergency bearer services is released either due to a radio link failure or a termination of the current emergency session, the MS and the network shall change the maximum uplink bit rate and the maximum downlink bit rate to 0 kbit/sec. Thus, the MS is unable to re-establish the IMS call for the emergency bearer service once the associated RAB is released since the MS is not allowed to perform a PDP context modification procedure to modify the associated bearer resources (i.e. the maximum uplink bit rate and the maximum downlink bit rate) for a PDP context for emergency bearer services.
In addition, according to 3GPP TS 24.008 V9.3.0, the MS is not required to retry a PDP context modification procedure or a Service Request procedure to re-establish a RAB for a PDP context with a conversational or streaming class, if:
the PDP Context Modification request invoked by the MS is accepted by the network but the radio access bearer is not established; or
the PDP Context Modification request is rejected with cause “insufficient resources”.
This impacts the emergency IMS service as the MS is not allowed to retry the PDP context modification procedure or the Service Request procedure to re-establish the RAB if the first try fails, and a user is unable to immediately get emergency IMS service.